Exemplary embodiments of the invention relate to a reducing agent addition and preparation system in a motor vehicle, for adding a liquid reducing agent to an exhaust gas emitted by an internal combustion engine of the motor vehicle and for preparing the added reducing agent in the exhaust gas.
For exhaust after treatment, in the initial state liquid reducing agents such as mineral fuel or aqueous urea solution are often introduced into the exhaust gas of internal combustion engines. With regard to preparation of the reducing agent introduced in the liquid state, there is a problem with respect to a desired uniform distribution and with respect to evaporation. For an aqueous urea solution, there is the additional problem of releasing the ammonia, required for the selective catalytic reduction of nitrogen oxides, by hydrolysis and/or thermolysis. Various exhaust gas system variants with preparation sections, mixers, evaporators, and hydrolysis catalytic converters have already been proposed for solving the problem. Despite the numerous proposed solutions, there is still a need for improvement with regard to the preparation of reducing agents that are introduced, in particular in the liquid state, into the exhaust gas.
Exemplary embodiments of the invention are, therefore, directed to a device by means of which advantageous introduction of reducing agent and the best possible preparation of reducing agent introduced into exhaust gas of an internal combustion engine is made possible.
The reducing agent addition and preparation system according to the invention for adding a liquid reducing agent to exhaust gas emitted by an internal combustion engine of the motor vehicle and for preparing the added reducing agent in the exhaust gas includes an injector associated with a reducing agent metering unit and which may spray a reducing agent spray jet into the exhaust gas, a mixing tube situated in an exhaust tract of the internal combustion engine and having a first and a second axial mixing tube end, and a funnel element situated on or in the mixing tube in the region of the first mixing tube end and which widens conically at a predetermined widening angle in the direction from the first mixing tube end toward the second mixing tube end. The mixing tube and the funnel element are designed as hollow bodies of rotation having a respective perforated lateral surface, and the perforation of the mixing tube is designed with an at least predominantly larger surface area than the perforation of the funnel element. On its end associated with the first mixing tube end, the funnel element has a passage opening, and the reducing agent metering unit is situated in such a way that the injector may spray the reducing agent spray jet through the passage opening into the interior of the funnel element.
Due to an inflow of exhaust gas at a comparatively high velocity into the interior of the funnel element, which is made possible by the perforation of the funnel element, the spray jet sprayed into the funnel element by the injector may develop into a fine spray mist in an optimal manner at that location while avoiding wall wetting. Further transport in the direction of the second mixing tube end is made possible, and, due to the larger perforation of the mixing tube and the resulting inflow of exhaust gas into the interior of the mixing tube at a comparatively low velocity, wall wetting is likewise at least largely avoided. A preparation section is thus provided that can form a fine spray of reducing agent droplets, which advantageously may evaporate in an increasingly easier manner on their transport path. Upon exiting the second mixing tube end, the reducing agent is therefore typically at least predominantly evaporated, and a homogeneous distribution over the entire cross section of the exhaust gas line is made possible.
In one embodiment of the invention, the rotational axes of the mixing tube and the funnel element at least approximately coincide, and the passage opening is situated at least approximately centrally with respect to the rotational axis. Due to this axially aligned, rotationally symmetrical arrangement, centered spraying of the reducing agent into the funnel element is made possible, which assists in avoiding wall wetting.
In another embodiment of the invention, the injector has an injector tip at the end, wherein the injector tip and the passage opening in the funnel element in each case have an at least approximately circular perimeter, and an outlet opening for the reducing agent is situated at least approximately centrally at the injector tip, and the outlet opening and a center point of the passage opening are situated at least approximately on the rotational axis of the funnel element, and the injector tip is situated opposite from the funnel element at a small distance from the passage opening in such a way that the injector tip and a lateral surface region of the funnel element surrounding the passage opening form an annular gap through which exhaust gas may flow into the interior of the funnel element. Due to the design of the annular gap, exhaust gas entering the annular gap and passing into the interior of the funnel element may envelop the spray jet emitted by the injector from behind and transport same in the axial direction essentially free of swirl and backflow. In addition, formation of flow-related dead zones, wall wetting, and resulting deposits are avoidable due to this embodiment. It is particularly advantageous when, in another embodiment of the invention, the annular gap is dimensioned in such a way that exhaust gas flowing through the annular gap into the funnel element has an inlet velocity, at least in operating points of the internal combustion engine above partial load, which is greater than a velocity at which the injector sprays the reducing agent spray jet. As a result, particularly effective formation of a fine spray due to finer distribution of droplets of reducing agent is made possible, and the evaporation behavior is improved.
In addition, it has proven to be particularly advantageous when, in another embodiment of the invention, the injector may spray an at least approximately conically expanding reducing agent spray jet into the exhaust gas, a widening angle of the reducing agent spray jet at least approximately corresponding to the widening angle of the funnel element. This allows a uniform finer division of the droplets of reducing agent while avoiding wetting of the inner lateral surface of the funnel element and forming uniform undisturbed flow conditions.
In another embodiment of the invention, the widening angle of the funnel element is in the range between 20 degrees and 70 degrees, in particular between 30 degrees and 60 degrees. This has proven to be particularly favorable for flow and advantageous for the preparation of reducing agent.
In another embodiment of the invention, due to the perforation of the funnel element, exhaust gas flowing into the funnel element has an at least approximately perpendicular orientation with respect to an enveloping surface of the reducing agent spray jet cone of the reducing agent spray jet sprayed by the injector. An exhaust gas cushion is formed that shields the inner wall of the funnel element, thus further reducing the risk of wall wetting and promoting fragmentation of the droplets of reducing agent.
In another embodiment of the invention, the funnel element is situated at least predominantly in the interior of the mixing tube, the funnel element at its widened end inwardly resting against the lateral surface of the mixing tube all the way around or being integrally joined and/or connected in a positive-fit manner to the mixing tube. This ensures a stable arrangement of the mixing tube and funnel element with respect to one another that is particularly compact.
In another embodiment of the invention, an exhaust gas guide element is provided that directs a portion of exhaust gas flowing in the direction of the first mixing tube end to the perforated lateral surface of the funnel element, and directs a second portion in the direction of a portion of the perforated lateral surface of the mixing tube that is downstream from the funnel element. This allows flow conditions that are uniform and largely independent of the flow velocity. The exhaust gas component that is directed in the direction of the perforated lateral surface of the funnel element and that at least partially flows into the interior of the funnel element has a facilitating effect for the formation of a fine reducing agent spray. The portion of exhaust gas that is directed in the direction of the lateral surface of the mixing tube and that at least partially flows into the interior of the mixing tube results in further transport of the reducing agent.
In another embodiment of the invention, for conducting exhaust gas an exhaust gas conducting element bent in the shape of a knee is provided with a first leg on the inflow side, and a second leg, adjoining the first leg, on the outflow side, the mixing tube extending from a deflection area from the first leg to the second leg into the second leg and being situated concentrically with respect to the second leg. This allows a particularly advantageous, compact arrangement of the overall reducing agent addition and preparation system. For simplifying the connection to the exhaust gas tract, it is also advantageously provided that a first separating point for attaching an exhaust gas supply pipe is provided on an inflow-side end of the first leg, and a second separating point for connecting an exhaust gas discharge pipe is provided on the outflow-side end of the second leg. According to another advantageous embodiment of the invention, the mixing tube extends beyond the second separating point from the exhaust gas conducting element bent in the shape of a knee. In this way a flow-related adverse effect of the separating point on the preparation of the reducing agent is avoidable, since the mixing tube shields the separating point.
In another embodiment of the invention, a swirl-forming plate part is provided for the exhaust gas supply pipe. Improvement in a uniform distribution of the exhaust gas flow with regard to the cross section of the exhaust gas supply pipe is thus made possible.
It has proven to be particularly advantageous when, in another embodiment of the invention, the exhaust gas conducting element bent in the shape of a knee has an exterior thermally insulating shell, at least in sections. Heat losses in the area of the exhaust gas tract which are critical for preparing the reducing agent, which hinder evaporation of the reducing agent, are thus largely avoided or at least reduced.
In another embodiment of the invention, a flange part is provided, to which the mixing tube and the reducing agent metering unit are fastened, and to which at least one cooling plate for dissipating heat from the flange part to the surroundings is also fastened, on the outer side with respect to the exhaust gas tract. This design allows a reduction in the heat stress on the overall metering unit and in particular the injector. It is particularly advantageous when a first and a second cooling plate are provided, the first and second cooling plates being in heat transfer contact with one another via a heat transfer plate, and being held at a distance from one another by the heat transfer plate. This allows further improvement in the heat dissipation, whereby the heat transfer plate also allows targeted shaping of the temperature gradients which form, and therefore high temperatures may be kept away from critical locations in this regard in a targeted manner.